Today, multi-chip, and other integrated circuit, modules rely on electrical conductors for connecting signal paths. Using these electrical conductors imposes many limits on the performance of these modules.
There are two primary limits related to communication. The first area of limit is communication speed. Electrical conductors have resistive and inductive characteristics, based on their physical dimensions. The maximum speed of communication will be limited because the conductor electrical characteristics will limit the speed at which the signal can be propagated. This is caused by the inductive characteristic of the conductor will resist the change in current flow as the signal changes state. Also, the resistive characteristic will limit the rate of current flow into the receiving circuit, typically capacitive in nature.
The second area of limit is commonly known as a communication race condition. This happens when a sending device communicates a signal to more than one receiving device through electrical conductors at unequal distances from the sending device. Combined with the electrical conductor characteristics earlier stated, the signal will arrive at the receiving devices at different times. This is a problem in systems relying on simultaneous operation of parallel circuits.
Another limit of electrical conductors is the electromagnetic interference effect. Electrical conductors act like antennas that both emit and receive unwanted electromagnetic signals. This causes improper device operation.
Electrical conductors also limit the size of a module because they take up space. This space includes the space needed for the physical electrical conductors and the space needed for insulation between electrical conductors. Also, sometimes, a clearance space is needed for an assembly machine to connect the electrical conductors. These space requirements make it difficult to minimize the size of the module.
Using electrical conductors limits the complexity of connections practical in a module. Some connection requirements are so complex that the designer must resort to multi-layered, or multi-dimensional, connections that adversely effect reliability and are difficult to manufacture. As modules get more functions, a mix of various analog and digital technologies are used. The electrical interface between these different technologies is sometimes difficult when using direct electrical connections because additional interface circuits are often necessary.
The inflexibility of electrical connections also limits system performance because the flexible substitution of redundant circuits is not possible. There are several reasons for wanting this flexibility; including the substitution of functioning circuits for faulty circuits to improve fault tolerant operation. Another reason for flexible substitution is for dynamic problem distribution in a system that breaks down a problem into small pieces for parallel solution, such as a parallel computer.
In summary, using electrical conductors for connecting signal paths between devices within an integrated circuit module is problematic. These problems include a limit to communication speed between devices and communication race conditions between more than two devices because of the physical characteristics of the electrical conductor. Other limits include faulty device operation because of electromagnetic wave interference, the difficulty of module size minimization, the practicality of complex connections, interface between different technologies, and the inflexibility of connections for circuit substitution.